Method and device for separating a mixture of fluids

ABSTRACT

A mixture of fluids is separated into at least two phases, one of which has a higher density than the other, passing the mixture through a normally horizontal supply pipe, by creating a stratified flow in the supply pipe, by passing the mixture through an inclined pipe, whilse maintaining a stratified flow in the inclined pipe, by extracting fluid with lower density (“lighter phase”) via a first discharge system and fluid with a higher density (“heavier phase”) via a second discharge system, wherein the interface between the lighter phase and the heavier phase is monitored in the inclined pipe by a level controller means that varies the flow of the fluid of higher density to keep the interface between set levels.

PRIORITY CLAIM

The present application claims priority on European Patent Application02252120.7 filed 25 Mar. 2002.

Field of the Invention

The present invention relates to a method and device for separating amixture of fluids, which are not completely mixable.

BACKGROUND OF THE INVENTION

Such mixtures appear, e.g., in the recovery of natural resources fromoil and gas wells. The products of such recoveries can include mixturesof gas, oil and water. These mixtures may comprise three phases.Depending on pressures and other physical circumstances the mixturesrecovered may comprise two phases, a hydrocarbon phase containing oiland optionally dissolved gas, and water. It also occurs that hardly anygas is present so that there is a two-phase mixture.

It is desirable to separate the water phase from the hydrocarbon phasebefore transporting the valuable products further. Thereto devices havebeen developed to separate such mixtures into the respective phases, andremoving the water phase.

Russian patent publication No. 2 077 364 discloses a device forseparating a mixture of fluids into three phases, gas, liquid of lowerdensity and a liquid of higher density, having a feed inlet, a gasoutlet, and outlets for the liquid phases. During operation of thisdevice, a mixture of gas, low-density liquid and high-density liquid issupplied to the feed inlet of the separation device. The mixture passesupwards through an upwardly inclined supply pipe towards the inlet of adownwardly inclined pipe. In the upper end of the downwardly inclinedpipe, gas is separated from the liquids that fall downwards towards thelower end of the downwardly inclined pipe. Gas, lighter liquid andheavier liquid are separately removed from the device via the respectiveoutlets.

A disadvantage of the known separation device is that turbulence isgenerated in the upwardly inclined feed pipe and that counter-currentflow prevails in the downwardly inclined pipe, which phenomena adverselyaffect the separation efficiency of the device.

European Patent Application No. 1044711 describes a device forseparating a mixture of fluids into three phases; gas, lower-densityliquid (“lighter liquid”) and higher-density liquid (“heavier liquid”),having a feed inlet, a gas outlet, an outlet for the lighter liquid andan outlet for the heavier liquid. The device comprises:

-   -   a normally horizontal supply pipe with the feed inlet at its        upstream end;    -   an inclined pipe having an inlet at its upper end that is        connected to the outlet of the supply pipe and having a closed        lower end;    -   a gas discharge system comprising a gas riser having an inlet        that is located in the gas-filled space and an outlet that is in        fluid communication with the gas outlet of the device;    -   a discharge system for lighter liquid having an inlet that is        located in the lighter liquid-filled space and an outlet that is        in fluid communication with the outlet for lighter liquid of the        device; and    -   a discharge system for heavier liquid having an inlet arranged        below the bottom level of the supply pipe and an outlet that is        in fluid communication with the outlet for heavier liquid of the        device,        wherein the diameter of the supply pipe is selected such that        during normal operation the velocities of the liquids are below        a pre-determined value, wherein the ratio of the length of the        supply pipe to its diameter is larger than 10, and wherein the        slope of the inclined pipe is selected such that during normal        operations a stratified flow is maintained in the inclined pipe.

One of the advantages of this device over the device of the Russianpatent is the maintenance of a stratified flow. However, the dischargesystems, especially the discharge system for the heavier liquid iscomplicated. Although the design can operate at a wide range ofoperating conditions, it does not provide for any active flow control.

SUMMARY OF THE INVENTION

The method of the present invention provides an easy way to ensure thatthe different phases are withdrawn separately from the separationdevice. The fact that the mixture is maintained in a stratified flow isa major enabler of this accomplishment. In one embodiment the supplypipe is different from the pipeline conduit through which the mixture isfed to the separation device described. This is suitably the case whenthe mixture is not in a stratified flow, and the supply pipe serves thento achieve such stratified flow. In an alternative embodiment, inparticular when the mixture is already in a stratified flow, the supplypipe is the same as the pipeline conduit. Applicant had found thatstratified flow in the supply conduit can be maintained if the diameterof the supply pipe is selected such that during normal operation thevelocities of the liquids are below a pre-determined value, and if theratio of the length of the supply pipe to its diameter is larger than 5,especially larger than 10. Applicant has further found that the slope ofthe inclined pipe can be selected such that during normal operations astratified flow is maintained in the inclined pipe. Suitably, the slopeof the inclined pipe ranges between 1 and 5, preferably between 1 and 3°from the horizontal plane. The stratified flow makes it possible to usea level monitor to determine where the interface between the heavier andthe lighter phases is. Via the first or second discharge system thelevel can then be adjusted. Preferably one employs a level controller incombination with a valve as the level controller means in the firstand/or second discharge system.

The second discharge system must be in fluid communication with theheavier phase in the inclined pipe. A skilled artisan will realize thatthe location of this discharge system can be along any place of theinclined pipe. Most conveniently, the second discharge system is at thedownstream end of the inclined pipe.

It is a further embodiment of the present invention to provide aseparation device that can be used in the current method. Accordingly,the present invention also provides a device for separating a mixture offluids into at least two phases, one of which has a higher density thanthe other, so that a phase of fluid of lower density (“lighter phase”)and a phase of fluid of higher density (“heavier phase”) are obtained,which device comprises:

-   -   a normally horizontal supply pipe with a feed inlet at its        upstream end and an outlet at its downstream end;    -   an inclined pipe having an inlet at its upper end that is        connected to the outlet of the supply pipe;    -   a first discharge system having an inlet that is located such        that is in fluid communication with the lighter phase; and    -   a second discharge system located at the inclined pipe and        having an inlet that is in fluid communication with the heavier        phase,        wherein the inclined pipe is provided with a level controller        means comprising a level monitor and a valve at the first and/or        second discharge system.

A skilled artisan will realise that many types of level monitors can beused. Examples of such level monitors include floating devices orsegmented monitors. Other examples are dip sticks, magnetic level liquidindicators, conductivity- or capacitance-based devices and ultrasoniclevel devices. An overview of suitable devices has been described inKirk-Othmer, Encyclopedia of Chemical Technology, John Wiley, New York,4th ed. Vol. 15, 1995, pp 409-433. The level monitors and the valves maybe operated via a computer-directed system.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described by way of example in more detailwith reference to the accompanying drawings, wherein

FIG. 1 shows schematically a first embodiment of the invention, and

FIG. 2 shows schematically a second embodiment of the invention.

DETAILED EMBODIMENT OF THE INVENTION

Reference is made to FIG. 1. A device 1 for separating a mixture offluids in two phases is shown. It contains a horizontal supply pipe 2.At the upstream side thereof it has an inlet that is connected to apipeline 3 that feeds the device with the mixture. It is noted that inthis embodiment the supply pipe 2 ensures a stratified flow that is notpresent in pipeline 3. At its downstream end it is connected to aninclined pipe 4. In the pipe 2 a stratified flow is created and ininclined pipe 4 such stratified flow is maintained so that two distinctphases are apparent; one lighter phase of fluid with a lower density anda heavier phase with a fluid of higher density. The device 1 alsocomprises a first discharge system 5, the inlet of which is in fluidcommunication with the lighter phase. The discharge systems results in adischarge pipe 6 through which the low-density fluid is discharged. Theinclined pipe 4 ends in a second discharge system 7 that boils down toan outlet through which the high-density fluid is passed into adischarge pipe 8. The interface between the two phases is monitored viaa level controller 9. If the level controller 9 indicates that theinterface goes beyond preset levels, it causes a valve 10 to adjust theflow of higher-density fluid through the discharge pipe 8, in order tobring the interface back to the preset level range. The valve 10 ispositioned in the discharge pipe 8. It is also possible to place thevalve at the outlet of the inclined pipe or at the lower end of theinclined pipe.

In an embodiment where the flow rate of higher-density fluid isrelatively high in comparison to the flow of the low-density fluid, itis suitable to extract the fluid of higher-density via a furtherdischarge system having a further outlet (11) that is in fluidcommunication with the heavier phase. Preferably, the further outlet 11is arranged in the bottom of the horizontal supply pipe. Thehigher-density fluid is then suitably withdrawn via the outlet 11through a discharge pipe 12. Suitably, this flow of higher-density fluidis combined with the higher-density fluid withdrawn from the seconddischarge system 7 in discharge pipe 8. It is preferred to have thevalve positioned in the second discharge system. An additional controlmeans is suitably arranged such that the pressure of the lighter phasein the first system is monitored, and the flow of fluid of lower densityis adjusted in accordance with the pressure measured. Although it ispossible to arrange the flow adjustment of the heavier phase inaccordance with the pressure measured, it is preferred from a pragmaticviewpoint to adjust the flow of low-density fluid.

The device and method are suitable employed in the production ofhydrocarbons, in which production also water is recovered. Hence thelighter phase in the present-method comprises suitably oil, and theheavier phase comprises suitably water. In a number of occasions thewell from which hydrocarbons are produced yield liquid hydrocarbon oil,water and gas. In such instances it is advantageous to separate thewater from the liquid and gaseous hydrocarbons. Thereto, the presentinvention suitably provides a method in which three phases are separatedinto a phase of fluid of lower density (“lighter phase”), a phase withfluid of intermediate density (“intermediate phase”) and a phase withfluid of higher density (“heavier phase”). The lighter phase suitablycomprises gas, the intermediate phase comprises oil and the heavierphase comprises water. Although in principle it is possible to withdrawthe combination of the intermediate and the heavier phases together viathe second discharge system, it is preferred to withdraw the lighterphase and the intermediate phase via the inlet of the first dischargesystem. Reference is made to FIG. 2, wherein the device 20 comprises ahorizontal supply pipe 21 and an inclined pipe 22. A mixture of gas, oiland water is fed to the device 20 via a pipe 23. A riser 24 is in fluidcommunication with the lighter and the intermediate phases, i.e. gas andoil. The lighter and intermediate phases are withdrawn from thehorizontal pipe, and the withdrawn phases are suitably passed to theriser section 24 to allow the lighter phase and the intermediate phaseto separate. The combined phases may be withdrawn from the riser section24 in any known manner. One can pass the combined phases directly to adischarge conduit (not shown). It is also possible to withdraw thelighter phase from the upper part of the riser section and theintermediate phase from a lower part of the riser section (not shown).It is preferred to provide a riser section that comprises two vessels,and pass the combination of the lighter and intermediate phases from afirst vessel 24 to a second vessel 25. Vessel 25 can have the shape of avessel as shown in the figure, but it will be clear that also othershapes, such as a short (horizontal) pipeline section can be used. Inthis second vessel 25 the phases clearly separate. The lighter phase isthen withdrawn from the top of the second vessel 25 via a conduit 27,and the intermediate phase is withdrawn from the bottom of the secondvessel 25 via a conduit 26. Suitably, the interface between the lighterphase and the intermediate phase is monitored by a second levelcontroller means 28 that adjusts the flow of the fluid of intermediatedensity to keep the interface between set levels using a valve 29arranged in the discharge pipe 26. Although not necessary, it issometimes economic to combine both phases from pipes 26 and 27 forfurther transport.

In this method the second level controller 28 is arranged at the secondvessel 25 to monitor the interface between the lighter and intermediatephases. Suitably, the level controller 28 communicates with a valve 39,arranged in the conduit 26 for the fluid of intermediate density, tocontrol the flow of fluid of intermediate density. The skilled artisanwill realise that it is possible to arrange a similar valve in thedischarge conduit 27 to control the flow of fluid of lighter density,and have that valve governed by the level controller 28. An alternativeembodiment is to have one or more similar level controllers communicatevalves in both discharge pipes to control both flows. It is furtheradvantageous to arrange a pressure monitor 29 at the discharge pipe 27that communicates with one or more flow control valves (30) in the firstdischarge system for control of the flow of the fluid of lower density.Alternatively, the pressure controller 29 may communicate with a valvein discharge pipe 26 to control the flow of fluid of intermediatedensity. A further embodiment provides communication of the pressurecontroller with two valves in pipes 26 and 27 to control both flows. InFIG. 2 only a valve 30 is shown in the discharge conduit 27 for thefluid of lower density. In this way the pressure in the riser section(24,25) is monitored and the flow of the fluid of lower density, or theflow of fluid of intermediate density or both flows are adjusted inaccordance with the pressure measured. The high-density fluid, e.g.water, is withdrawn from the inclined pipe 22 via a discharge pipe 31.The level between the intermediate and heavier phases is monitored by alevel controller 32 that is in communication with a valve 33, arrangedin discharge pipe 31. The system of level controller 32 and valve 33operates similarly to the equivalent system described for FIG. 1.

The following is an example of the separation device wherein referenceis made to the embodiment of FIG. 2.

A mixture of gas, oil and water is fed via a tube of 10″ (254 mm)internal diameter to the separation device 20. The horizontal supplypipe 21 and the inclined pipe 22 have an internal diameter of 48″ (1.2m). Therefore the superficial liquid velocity in the device is reducedby a factor of about 24. The length of the horizontal supply pipe andthat of the inclined pipe may range from 12 to 100 m, and from between15 and 120 m, respectively. It is again emphasised that the length ofthe supply pipe is not relevant as long as a stratified flow is createdin the supply pipe. If the mixture is fed through a pipeline conduit ina stratified flow, the supply pipe can be as long as many tens ofkilometres. The temperature of the mixture varies between 60 to 85° C.and the pressure may be varied between 25 and 85 bar. Suitably the levelmonitor is in the middle of the inclined section. That could be at about10 m downstream of the riser section. Under the conditions describedabout 10,000 to 20,000 m³ of the mixture per day can be passed throughthe separation device.

While the illustrative embodiments of the invention have been describedwith particularity, it will be understood that various othermodifications will be readily apparent to, and can be easily made by oneskilled in the art without departing from the spirit of the invention.Accordingly, it is not intended that the scope of the following claimsbe limited to the examples and descriptions set forth herein but ratherthat the claims be construed as encompassing all features which would betreated as equivalents thereof by those skilled in the art to which thisinvention pertains.

1. A method for separating a mixture of fluids into at least two phases,one of which has a higher density than the other, which methodcomprises: passing the mixture through a normally horizontal supply pipewith a feed inlet at its upstream end and an outlet at its downstreamend; subsequently, passing the mixture through an inclined pipe havingan inlet at its upper end that is connected to the outlet of the supplypipe, whilst maintaining a stratified flow in the inclined pipe so thatthe phase of fluid of lower density (“lighter phase”) is above the phaseof fluid of higher density (“heavier phase”); extracting fluid with thelower density via a first discharge system having an inlet that islocated such that is in fluid communication with the lighter phase;extracting fluid of higher density via a second discharge system locatedat the inclined pipe and having an inlet that is in fluid communicationwith the heavier phase; and wherein the interface between the lighterphase and the heavier phase is monitored in the inclined pipe by a levelcontroller means that varies the flow of the fluid of higher density tokeep the interface between set levels; wherein the fluid of higherdensity is extracted via a further discharge system having a secondoutlet that is in fluid communication with the heavier phase; whereinthe second outlet is arranged in the bottom of the horizontal supplypipe.
 2. The method of claim 1, wherein the level controller meanscomprises a level monitor and a valve at the first and/or seconddischarge system.
 3. The method of claim 1, wherein the supply pipe isthe same as the pipeline conduit through which the mixture to beseparated is fed.
 4. The method of claim 1, wherein the pressure of thelighter phase in the first discharge system is monitored, and the flowof fluid of lower density is adjusted in accordance with the pressuremeasured.
 5. The method of claim 1, wherein the lighter phase comprisesoil, and the heavier phase comprises water.
 6. The method of claim 1,wherein a third phase with fluid of intermediated density(“intermediated phase”) is further seprated.
 7. The method of claim 6,wherein the lighter phase comprises gas, the intermediate phasecomprises oil and heavier phase comprises water.
 8. The method of claim6, wherein the lighter phase and the intermediate phase are withdrawnvia the inlet of the first discharge system.
 9. The method of claim 6,wherein the withdrawn phases are passed to a riser section to allow thelighter phase and the intermediate phase to separate.
 10. The method ofclaim 9, wherein the lighter phase is withdrawn from the riser sectionat the upper level of the riser section, and the intermediate phase iswithdrawn from the riser section at the lower level of the risersection.
 11. The method of claim 9, wherein the riser section comprisesa first vessel in communication with said horizontal supply pipe and asecond vessel communication with said first vessel.
 12. The method ofclaim 9, wherein the interface between the lighter phase and theintermediate phase is monitored by a second level controller means inthe riser section that adjusts the flow of the fluid of lower density tokeep the interface between set levels.
 13. The method of claim 12,wherein the second level controller means communicates with valves thatcontrol the flow of the fluid of lower density or the flow of the fluidof intermediate density or both flows.
 14. The method of claim 6,wherein the pressure in the riser section is monitored and the flow ofthe fluid of lower density, or the flow of fluid of intermediate densityor both flows are adjusted in accordance with the pressure measured. 15.A device for separating a mixture of fluids into at least two phases,one of which has a higher density than the other, so that a phase offluid of lower density (“lighter phase”) and a phase of fluid of higherdensity (“heavier phase”) are obtained, which device comprises: anormally horizontal supply pipe with a feed inlet at its upstream endand an outlet at its downstream end; an inclined pipe having an inlet atits upper end that is connected to the outlet of the supply pipe; afirst discharge system having an inlet that is located such that is influid communication with the lighter phase; a second discharge systemlocated at the inclined pipe and having an inlet that is in fluidcommunication with the heavier phase, wherein the inclined pipe isprovided with a level controller means comprising a level monitor and avalve at the first and/or second discharge system and; a furtherdischarge system having a second outlet that is in fluid communicationwith the heavier phase; wherein the second outlet is arranged in thebottom of the horizontal supply pipe.
 16. The device of claim 15,wherein the first discharge system is provided with a pressure monitorthat communicates with a flow control valve located in the firstdischarge system.
 17. The device of claim 15, wherein the firstdischarge system comprises a riser section to allow the lighter phaseand a phase with fluid of intermediate density (“intermediate phase”)with a density higher than the density of the lighter phase but lowerthan the density of the heavier phase to separate, which riser sectionhas an outlet for the lighter phase in the upper part and an outlet forthe intermediate phase in the lower part.
 18. The device of claim 17,wherein the riser section comprises a first vessel in communication withsaid horizontal supply pipe and a second vessel in communication withsaid first vessel.
 19. The device of claim 17, wherein the riser sectionhas been provided with a second level controller means that adjusts theflow of the fluid of lower density to keep the interface between setlevels.
 20. The device of claim 19, wherein the second level controllermeans communicates with one or more valves at the respective dischargesystems, which valve(s) control(s) the flow of the fluid of lowerdensity or the flow of the fluid of intermediate density or both flows.21. The device of claim 17, wherein the riser section is provided with apressure monitor that communicates with one or more flow control valvesfor control of the flow of the fluid of lower density, or the flow offluid of intermediate density of both flows.